To provide cellular wireless communication service, a wireless service provider or “wireless carrier” typically operates a radio access network (RAN) that defines one or more coverage areas in which wireless communication devices (WCDs) can be served by the RAN and can thereby obtain connectivity to broader networks such as the public switched telephone network (PSTN) and the Internet. A typical RAN may include one or more base transceiver stations (BTSs) (e.g., macro network cell towers and/or femtocells), each of which may radiate to define a cell and cell sectors in which WCDs can operate. Further, the RAN may include one or more radio network controllers (RNCs) or the like, which may be integrated with or otherwise in communication with the BTSs, and which may include or be in communication with a switch or gateway that provides connectivity with one or more transport networks. Conveniently with this arrangement, a cell phone, personal digital assistant, wirelessly equipped computer, or other WCD that is positioned within coverage of the RAN can then communicate with a BTS and in turn, via the BTS, with other served devices or with other entities on the transport network.
Wireless communications between a WCD and a serving BTS in a given coverage area will typically be carried out in accordance with an agreed air interface protocol that defines a mechanism for wireless exchange of information between the WCD and BTS. Examples of such protocols include CDMA (e.g., 1xRTT, 1xEV-DO), iDEN, WiMAX (e.g., IEEE 802.16), LTE, TDMA, AMPS, GSM, GPRS, UMTS, or EDGE, and others now known or later developed.
The air interface protocol will generally define a “forward link” encompassing communications from the BTS to WCDs and a “reverse link” encompassing communications from WCDs to the BTS. Further, each of these links may be structured to define particular channels, through use of time division multiplexing, code division multiplexing (e.g., spread-spectrum modulation), frequency division multiplexing, and/or some other mechanism.
The forward link, for example, may define (i) a pilot channel on which the RAN may broadcast a pilot signal to allow WCDs to detect wireless coverage, (ii) system parameter channels (e.g., a sync channel) on which the RAN may broadcast system operational parameters for reference by WCDs so that the WCDs can then seek network access, (iii) paging channels on which the RAN may broadcast page messages to alert WCDs of incoming communications, and (iv) traffic channels on which the RAN may transmit bearer traffic (e.g., application data) for receipt by WCDs. And the reverse link, for example, may define (i) access channels on which WCDs may transmit “access probes” such as registration messages and call origination requests, and (ii) traffic channels on which WCDs may transmit bearer traffic for receipt by the RAN.
Certain air interface channels may be arranged to carry communications in a time division multiplexed manner, so that multiple WCDs can share the channel. As such, those channels may have a limited capacity in the form of limited number of time slots that can be allocated for use by WCDs. Through engineering design, taking into account expected use, a sufficient number of such channels and sufficient channel capacity may be provided. However, in some situations, a communication channel can nevertheless become overloaded, which can unfortunately create issues with network performance.
The reverse link access channel is a good example of this issue.
Under certain air interface protocols, an access channel in each coverage area is divided into series of time slots that can be used to carry access probes so as to allow WCD registration, call origination, and other functionality. In typical practice, when a WCD is going to send an access probe to the BTS, the WCD will randomly select one of the access channel time slots and send the access probe in that time slot. Normally, the random nature of this selection will minimize the chance that two WCDs in the coverage area will transmit access probes at the same time (in the same time slot). However, at times when the coverage area is in heavy use, there is an increased chance that two or more WCDs will transmit access probes at the same time. When that happens, the access probes may collide with each other, and as a result neither access probe may successfully reach the BTS. If this problem persists, a WCD may ultimately conclude that it is unable to communicate with the BTS, which may lead to roaming and poor user experience.